The present invention relates generally to human vision testing devices and methods, and more particularly to a novel vision test chart and method for testing the ability of a subject to perceive contrast, and more specifically to perceive contrast as a function of size.
Numerous visual test pattern configurations and methods for testing the ability of a subject to visually distinguish shapes, sizes and contrasts have been proposed. Typically, these patterns have been characterized by variations of size and contrast of shapes having substantially well defined edges, such as Snellen charts, tumbling "E" arrays, Landolt "C" charts, etc. Certain of the prior art patterns are aperiodic, such as disks, rectangles, diamonds, etc.; others are quasi-periodic, such as tri-bar and tumbling "E" arrays, and small checkerboard designs; still others are periodic, such as square waves and sine waves. Only the pattern comprising a sine wave has been characterized by spatially varying luminance as an exception to the stated groups of shapes otherwise characterized by clearly defined edges. Sine wave patterns do not have spatial characteristics like visual objects in the real world, but do have "fuzzy edges", somewhat similar to real objects of varying luminance without sharply defined edges. An advantage of aperiodic patterns is that they require only a small area for the shape comprising the pattern, and may thereby be made substantially similar to real objects. In the prior art, therefore, the expedient has been to test a "single" spatial frequency; in so doing, relatively large size patterns of about three degrees of arc and larger have generally been utilized.
The present invention substantially eliminates the aforementioned problems in the prior art by providing a superior vision test chart and method for rapidly and reliably testing a subject for ability to perceive contrast, and more specifically to perceive contrast as a function of size. The chart and method of the invention include the advantages of both periodic and aperiodic targets without the attendant disadvantages. The novel visual test chart of the invention comprises an array of visually contrasting test shapes or spots of various sizes and contrasts on a contrasting background, each shape being characterized by a Gaussian luminance (or reflectance) distribution across the width of the shape. Each visual test shape of the chart has a reasonably bounded spatial frequency spectrum (i.e., no sharp edges). The array of the chart may preferably comprise rows and columns, wherein the shapes systematically vary in size within each row and in maximum contrast within each column. The method taught herein provides rapid and comprehensive visual contrast perception measurement and allows rapid mathematical analysis of the test results, since Gaussian distributions may be conveniently manipulated analytically.
It is, therefore, a principal object of the present invention to provide an improved method for rapidly testing the ability of a subject to perceive contrasts.
It is a further object of the invention to provide an improved contrast vision test chart.
These and other objects of the invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.